1. Field of the Invention
The present invention relates generally to micro-electromechanical (MEMS) devices and in particular to MEMS micro mirrors and a method for controlling the angle of deflection of a micro mirror.
2. Description of Related Art
FIG. 1 illustrates a mirror-in-frame design of a mirror pixel device 10. The device incorporates a mirror 12 which is supported by a frame 14 that forms gimbal structure 16. The device includes a pair of pivots 18, 20, one each for enabling movement in each axis of rotation. The pivots may include torsional springs that provide a restoring force for the mirror plate in a desired position. The position of the mirror is determined by the angle of the mirror within the frame and the angle of the frame with respect to the support of the gimbaled structure. The term position detection of the mirror as used in the specification should be interpreted to include both mirror and frame where appropriate.
The mirror and frame may include one or more thin electrode(s) on its surface. As shown in FIG. 1, the mirror has two electrodes 12a one on each lateral side of the pivot 18. The frame has two electrodes 14a one on each lateral side of the pivot 20. The mirror may be constructed of silicon. The electrode cooperates with an electrode on the surface of a substrate (not shown) and will move the mirror in response to the imposition of a voltage charge between the plates. Various designs have been proposed with more than one electrode on each of the mirror and frame. A pivot spring is typically used to urge the mirror back to a resting position once the charge is discontinued. Often, the mirrors are arranged in arrays with approximately 16 mirrors by 16 mirrors. Depending on any particular application, more or fewer mirrors can be arranged in an array.
The MEMS mirrors can be batch fabricated in a high density array configuration within a few micrometers of tolerance. The mirrors are typically moved along the pivots by electrostatic, electromagnetic, piezoelectric actuation, stepper motors, or thermal bimorphs. The MEMS mirrors can be used to steer a light beam in free space. Optical switches, for example, use MEMS mirrors to steer light into a desired direction. In optical switching applications the beam pointing stability is an important parameter that affects the overall system performance. If the pointing angle stability is not sufficient, significant losses, such as port-to-port losses, can occur and the overall performance may be compromised. Achieving a high-precision stable performance typically requires a servo system that can accommodate system noise and uncertainties.
Achieving a highly stable micro mirror has typically required a servo system that controls the actuation of the micro mirror and a control system to control the mirror deflection. Optical feedback control is one type of control system that has been proposed. This type of control system uses the telecommunication beam in an optical switch and the mirror is controlled by maximizing the optical power of a collimated optical beam reflected from the mirror and received in an optical fiber with photo tabs. Other example of optical feedback control uses a Position Sensing Detector (PSD) or a CCD camera to detect the position of a light beam reflected from the mirror.
Another control system that has been proposed includes adding piezoresistive deflection sensors to the suspension pivot beams of the inner mirror and the outer frame. The output of the angle sensors is a measure of deflection around the two axes of rotation and is used to control the servo mechanisms that control the angle of deflection of the mirror. One drawback to the use of piezoresistive angle sensors are temperature sensitive and require additional connections between the substrate and the control system. Additionally, a device that includes piezoresistive angle sensors may be more complicated and more difficult to manufacturing. Further, each of the above mentioned control systems add to the level of complexity and cost of a MEMS device. The control systems also occupy space and require additional connections between the MEMS device and the control system.
The area of study in which control systems are developed and analyzed is called motion control. Motion control theory has developed sophisticated analyses to define motion control systems which can be adapted to a closed feed back control system. The theory of Variable Structure Control (VSC) uses conventional control techniques and the response of a closed loop feedback system is determined by the control system in combination with the apparatus under control. Changes in the characteristics of the apparatus or disturbance forces acting on it will alter the dynamic response of the system and has limited the application of VSC systems. One subclass of VSC systems which does not have these disadvantages is Sliding mode (SLM) control. SLM control has the advantage that the response to the closed loop system is defined by parameters in the controller and is independent of both changes in the controlled apparatus and disturbances acting upon it.
SLM and VSC control systems are defined using phase space diagrams. The response of any system can be completely described by plotting the phase variables on a phase space diagram. The phase variables consist of the variable of interest (position) and its derivatives (velocity and acceleration). The number of states, or derivatives, required is determined by the order of the plant. For a second order position control system, a step change in control input produces a step change in acceleration and the dynamics of the plant are completely defined the position and velocity. Thus the phase space has two dimensions with position and velocity as the x and y axes. For position control of the third order, a step change in the control input causes a step change in jerk (the rate of change of acceleration). The phase space thus has three dimensions with the axes being position, velocity and acceleration. Sliding mode control has been used in devices such as compact disk drives and other areas.
Sliding mode control is an effective control technique that achieves precision tracking control and, in addition, yields considerable stability and performance robustness even with parameter changes and noise. SMC is desirable because it is regarded as simple and easily implemented. Simply, SMC uses a high frequency digital control that switches between two predetermined values of control inputs, e.g., on/off or +V/−V, on the basis of the error between the desired value, i.e., set point, of a performance output signal and its actual value. Specifically, the digital control signal switches on the basis of a sign of a certain function of the error and its derivatives. This function is referred to as the sliding mode function or simply sliding mode, and is the sum of the error signal and amplified derivative of the error signal. The amplification gain for the derivative is referred to as the differential gain or “Dgain”.
The differential gain in SMC must be sufficiently high to yield the desired amount of damping and stability. A lower differential gain can result in the over shooting the desired set point or even instability. A very high differential gain results in an over-damped slow tracking response. However the derivative term is a major source of output noise and tracking error. This effect is further amplified by a high differential gain. This tradeoff makes the implementation of sliding mode control sub-optimal in some high precision set-point tracking applications. In situations where implementation space is limited and feedback and computation loop delays are critical, there is a need for a more precise and robust control system.
The accurate and predictable control of MEMS devices has been elusive. The present invention seeks to provide a MEMS device which had accurate and predictable control. Further, the control provided by the present invention is less temperature sensitive that other control modes previously considered. Additionally, it is desirable to increase the range of controllable motion so that a MEMS mirror assembly may function in a range of applications.